Acute Myeloid Leukemia (AML) is one of the most common forms of leukemia in adults and despite advances in treatment the 5 year survival is still less than 50% in adults and significantly lower in the elderly. In fact, the median survival in patients over the age of 56 is less than one year and only 20% of these patients survive two years. Though the prognosis for younger patients is significantly better, disease-free survival at 6 years following complete remission is still only 40% in children and young adults. There is an enormous unmet need for novel therapeutics to improve the morbidity and mortality of these patients. This unmet need is particularly high in the elderly who often cannot tolerate traditional chemotherapy due to toxicities. Though there have been a number of clinical trials, there has been little improvement in overall survival in this age group over the last 30 years.
Acute myeloid leukemia is a broad range of disorders that are all characterized by leukemic cells that have a differentiation arrest. AML can be classified morphologically according to the French-American-British criteria by the degree of differentiation as well as extent of cell maturation as M0-M7. Treatment for all subtypes of AML is very similar, except for acute promyelocytic leukemia (APL, M3 subtype). Traditional therapy involves combination systemic chemotherapy. Several different approaches are utilized; however, they usually involve an induction therapy with cytarabine and a second chemotherapeutic such as daunorubicin or idarubicin and consolidation therapy with either a bone marrow transplant or additional chemotherapy. Besides significant side effects from the traditional chemotherapeutics, the efficacy of these agents in treating AML is poor.
To date the only exception to the poor treatment options for AML is the remarkable success of all-trans-retinoic acid ATRA for one relatively uncommon subtype (5-10% of AML), acute promyelocytic leukemia (APL). Utilizing a combination of ATRA and chemotherapy, the long term survival and presumed cure of 75-85% of patients is possible. ATRA illustrates the great promise for new agents with greater efficacy and less toxicity. In fact, elderly patients with APL who cannot tolerate traditional chemotherapy can achieve complete remission with therapies that utilize ATRA.
ATRA's success stems from the fact that AML is a clonal disease characterized by the arrest of differentiation of immature myeloid cells. ATRA overcomes this block in differentiation by forcing leukemic cells to terminally differentiate so that they are no longer capable of dividing. ATRA is successful in APL due to its ability to reverse the dominant negative effects of the PML-RAR fusion protein created by a chromosomal translocation, classically t(15;17)(q22;q21). This fusion protein interacts with the retinoid x receptor (RXR), nuclear corepressors and histone deacetylase (HDAC) resulting in repression of transcription that leads to the block in differentiation. At pharmacologic doses, ATRA is able to overcome the repression of transcription and differentiation results. Unfortunately, APL is a rare subtype of AML and ATRA has not been found to be clinically useful for other subtypes.
Though many compounds have been shown to have some differentiation-inducing effects in vitro, their clinical utility has been limited by either suboptimal differentiation-inducing capacity and/or toxicity. For example, Vitamin D3 induces potent differentiation, however, it also causes severe hypercalcemia at the required dose. Treatments that promote the differentiation of immature myeloid cells hold considerable promise in improving the long term survival of AML patients while avoiding some of the toxicities of traditional chemotherapy. Treatment of leukemia could be revolutionized by novel compounds due to their potential to cure leukemia and provide elderly patients with alternative nontoxic regimens.